A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen, an alcohol such as methanol or ethanol, or formic acid, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.
Fuel cells are usually classified according to the nature of the electrolyte employed. Often the electrolyte is a solid polymeric membrane, in which the membrane is electronically insulating but ionically conducting. In the proton exchange membrane fuel cell (PEMFC) the membrane is proton conducting, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water.
A principal component of the PEMFC is the membrane electrode assembly (MEA), which is essentially composed of five layers. The central layer is the polymer ion-conducting membrane. On either side of the ion-conducting membrane there is an electrocatalyst layer, containing an electrocatalyst designed for the specific electrolytic reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer which is porous and electrically conducting and allows the reactants to reach the electrocatalyst layer and conduct the electric current that is generated by the electrochemical reactions.
Conventional ion-conducting membranes used in PEMFCs are generally formed from sulphonated fully-fluorinated polymeric materials (often generically referred to as perfluorinated sulphonic acid (PFSA) ionomers). As an alternative to PFSA type ionomers, it is possible to use ion-conducting membranes based on partially fluorinated or non-fluorinated hydrocarbon sulphonated or phosphonated polymers. Recent developments in PEMFCs require membranes to be thinner (<50 μm) and of higher ion exchange capacity (IEC) or lower equivalent weight (EW) due to the advantages obtained (improved ionic conductivity, improved water transport etc) and thus, in order to provide the mechanical properties required to increase resistance to premature failure, a reinforcement, typically expanded polytetrafluoroethylene (ePTFE), is embedded within the membrane.
Although such reinforced membranes often have lower proton conductivity when compared to an unreinforced membrane of the same thickness, the improvement in mechanical properties has enabled thinner membranes of lower electrical resistance to be used.
Other types of reinforcement have also been proposed, for example as disclosed in WO2011/149732.